Electro-wetting on dielectric (EWOD) is a well-known technique for manipulating droplets of fluid by application of an electric field. The structure of a conventional EWOD device is illustrated in the cross section diagram of FIG. 1. As shown, the EWOD device includes a lower substrate 30 and an upper substrate 36 arranged opposite the lower substrate 30 and separated from it by a spacer 32 to form a fluid gap 35.
A conductive material is formed on the lower substrate 30 and patterned to form a plurality of individually addressable lower electrodes 38, as depicted in FIG. 1 for example as a first lower electrode 38A and a second lower electrode 38B. An insulator layer 20 is formed on the lower substrate 30 over the lower electrodes 38 and a lower hydrophobic coating 16 is formed over the insulator layer. The hydrophobic coating is formed from a hydrophobic material. The hydrophobic material is commonly, but not necessarily, a fluoropolymer. A conductive material is formed on the upper substrate 36 and acts as a common reference electrode 28. An upper hydrophobic coating 26 is formed over the common reference electrode 28. The fluid gap is filled with a non-polar filler fluid 34, such as oil, and liquid droplets 4. The liquid droplet 4, commonly an aqueous and/or ionic fluid, includes a polar material and is in contact with both the lower hydrophobic coating 16 and the upper hydrophobic coating 26. The interface between the liquid droplet 4 and filler fluid 34 forms a contact angle θ 6 with the surface of the lower hydrophobic coating 16.
In operation, voltage signals are applied to the lower electrodes 38 and common reference electrode 28 so as to actuate the liquid droplet 4 to move within the fluid gap 35 by the EWOD technique. Typically, the lower electrodes 38 are patterned to form an array, or matrix, with each element of the array comprising a single individually addressable lower electrode 38. A plurality of droplets may therefore be controlled to move independently within the fluid gap 35 of the EWOD device.
U.S. Pat. No. 6,565,727 (Shenderov, issued May 20, 2003) discloses an EWOD device with a passive type array for moving droplets.
U.S. Pat. No. 6,911,132 (Pamula et al., issued Jun. 28, 2005) discloses an EWOD device with a two dimensional array to control the position and movement of droplets in two dimensions.
U.S. Pat. No. 8,815,070 (Wang et al., issued Aug. 26, 2014) describes an EWOD device in which multiple micro-electrodes are used to control the position and movement of a droplet.
U.S. Pat. No. 8,173,000 (Hadwen et al, issued May 8, 2012) discloses an EWOD device with improved reliability by means of application of an AC voltage signal to the common reference electrode.
Active Matrix EWOD (AM-EWOD) refers to implementation of EWOD in an active matrix array incorporating transistors within each element of the array. The transistors may be, for example, thin film transistors (TFTs), and form an electronic circuit within each array element to control the voltage signals applied to the lower electrodes.
U.S. Pat. No. 7,163,612 (Sterling et al., issued Jan. 16, 2007) describes how TFT based thin film electronics may be used to control the addressing of voltage pulses to an EWOD array by using circuit arrangements very similar to those employed in active matrix display technologies.
U.S. Pat. No. 8,653,832 (Hadwen et al, issued Feb. 18, 2014) discloses an AM-EWOD device in which each element in the array includes circuitry to both control the voltage signals applied to the lower electrode and to sense the presence of a liquid droplet above the electrode.
A disadvantage of the conventional EWOD device structure as described above is the presence of an electric field across the upper hydrophobic coating 26. The hydrophobic coating is necessary for successful application of the EWOD technique but acts as an insulating material thus separating the common reference electrode 28 from electrical contact with the liquid droplet 4. Accordingly, the electric potential of the liquid droplet 4 may assume a different value from that applied to the common reference electrode 28. This potential difference and the resulting electric field present across the upper hydrophobic coating has been observed as a source of deleterious effects including reduction of the electrowetting force, the generation of bubbles in the filler fluid and degradation of chemicals or biological materials within the droplet. A method to reduce the electric field within the upper hydrophobic coating is therefore sought.